diff --git a/include/patty/ax25/frame.h b/include/patty/ax25/frame.h
index 913af67..0311a19 100644
--- a/include/patty/ax25/frame.h
+++ b/include/patty/ax25/frame.h
@@ -24,6 +24,23 @@ enum patty_ax25_frame_type {
     PATTY_AX25_FRAME_UNKNOWN    = 0xff
 };
 
+enum patty_ax25_frame_flags {
+    PATTY_AX25_FRAME_POLL  = (1 << 4),
+    PATTY_AX25_FRAME_FINAL = (1 << 4)
+};
+
+enum patty_ax25_frame_cr {
+    PATTY_AX25_FRAME_COMMAND,
+    PATTY_AX25_FRAME_RESPONSE
+};
+
+enum patty_ax25_frame_s_type {
+    PATTY_AX25_FRAME_S_RR   = 0x01,
+    PATTY_AX25_FRAME_S_RNR  = 0x05,
+    PATTY_AX25_FRAME_S_REJ  = 0x09,
+    PATTY_AX25_FRAME_S_SREJ = 0x0d
+};
+
 enum patty_ax25_frame_u_type {
     PATTY_AX25_FRAME_U_SABME = 0x6f,
     PATTY_AX25_FRAME_U_SABM  = 0x2f,
@@ -36,16 +53,6 @@ enum patty_ax25_frame_u_type {
     PATTY_AX25_FRAME_U_TEST  = 0xe3
 };
 
-enum patty_ax25_frame_u_flags {
-    PATTY_AX25_FRAME_U_POLL  = (1 << 4),
-    PATTY_AX25_FRAME_U_FINAL = (1 << 4)
-};
-
-enum patty_ax25_frame_cr {
-    PATTY_AX25_FRAME_COMMAND,
-    PATTY_AX25_FRAME_RESPONSE
-};
-
 typedef struct _patty_ax25_frame {
     patty_ax25_addr dest,
                     src,
diff --git a/src/server.c b/src/server.c
index 1e2459d..602e500 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1026,7 +1026,7 @@ error_toobig:
 
 static int reply_dm(patty_ax25_if *iface,
                     patty_ax25_frame *frame,
-                    enum patty_ax25_frame_u_flags flags) {
+                    enum patty_ax25_frame_flags flags) {
     patty_ax25_frame reply = {
         .control = PATTY_AX25_FRAME_U_DM | flags,
         .proto   = PATTY_AX25_PROTO_NONE,
@@ -1039,7 +1039,7 @@ static int reply_dm(patty_ax25_if *iface,
 
 static int reply_ua(patty_ax25_if *iface,
                     patty_ax25_frame *frame,
-                    enum patty_ax25_frame_u_flags flags) {
+                    enum patty_ax25_frame_flags flags) {
     patty_ax25_frame reply = {
         .control = PATTY_AX25_FRAME_U_UA | flags,
         .proto   = PATTY_AX25_PROTO_NONE,
@@ -1057,7 +1057,20 @@ static int reply_ua(patty_ax25_if *iface,
 
 static int reply_frmr(patty_ax25_if *iface,
                       patty_ax25_frame *frame,
-                      enum patty_ax25_frame_u_flags flags) {
+                      enum patty_ax25_frame_flags flags) {
+    patty_ax25_frame reply = {
+        .control = PATTY_AX25_FRAME_U_FRMR | flags,
+        .proto   = PATTY_AX25_PROTO_NONE,
+        .info    = NULL,
+        .infolen = 0
+    };
+
+    return reply_to(iface, frame, &reply, PATTY_AX25_FRAME_NORMAL);
+}
+
+static int reply_rej(patty_ax25_if *iface,
+                     patty_ax25_frame *frame,
+                     enum patty_ax25_frame_flags flags) {
     patty_ax25_frame reply = {
         .control = PATTY_AX25_FRAME_U_FRMR | flags,
         .proto   = PATTY_AX25_PROTO_NONE,
@@ -1077,7 +1090,7 @@ static int handle_sabm(patty_ax25_server *server,
 
     if ((local = sock_by_addr(server->socks_pending_accept,
                               &frame->dest)) == NULL) {
-        return reply_dm(iface, frame, PATTY_AX25_FRAME_U_FINAL);
+        return reply_dm(iface, frame, PATTY_AX25_FRAME_FINAL);
     }
 
     if ((client = client_by_sock(server, local)) < 0) {
@@ -1110,7 +1123,7 @@ static int handle_sabm(patty_ax25_server *server,
         goto error_sock_delete_by_addr;
     }
 
-    if (reply_ua(iface, frame, PATTY_AX25_FRAME_U_FINAL) < 0) {
+    if (reply_ua(iface, frame, PATTY_AX25_FRAME_FINAL) < 0) {
         goto error_reply_ua;
     }
 
@@ -1152,7 +1165,7 @@ static int handle_ua(patty_ax25_server *server,
     if ((sock = sock_by_addrpair(server->socks_pending_connect,
                                  &frame->dest,
                                  &frame->src)) == NULL) {
-        return reply_frmr(iface, frame, PATTY_AX25_FRAME_U_FINAL);
+        return reply_frmr(iface, frame, PATTY_AX25_FRAME_FINAL);
     }
 
     if ((client = client_by_sock(server, sock)) < 0) {
@@ -1253,7 +1266,7 @@ static int handle_info(patty_ax25_server *server,
          * TODO: Figure out how to respond to a packet sent outside of an
          * active session
          */
-        goto done;
+        return 0;
     }
 
     if (sock->status != PATTY_AX25_SOCK_ESTABLISHED) {
@@ -1261,17 +1274,19 @@ static int handle_info(patty_ax25_server *server,
          * TODO: Figure out how to respond to a packet sent to a socket not
          * in an established state
          */
-        goto done;
+        return 0;
     }
 
     /*
      * TODO: Validate RX and TX sequence numbers
      */
+    if (sock->seq_recv == PATTY_AX25_CONTROL_SEQ_SEND(frame->control)) {
+        sock->seq_recv++;
+    } else {
+        return reply_rej(iface, frame, PATTY_AX25_FRAME_FINAL);
+    }
 
     return write(sock->fd, frame->info, frame->infolen);
-
-done:
-    return 0;
 }
 
 static int handle_disc(patty_ax25_server *server,
@@ -1282,14 +1297,14 @@ static int handle_disc(patty_ax25_server *server,
     if ((sock = sock_by_addrpair(server->socks_established,
                                  &frame->dest,
                                  &frame->src)) == NULL) {
-        return reply_frmr(iface, frame, PATTY_AX25_FRAME_U_FINAL);
+        return reply_frmr(iface, frame, PATTY_AX25_FRAME_FINAL);
     }
 
     if (sock_close(server, sock) < 0) {
         goto error_sock_delete;
     }
 
-    return reply_dm(iface, frame, PATTY_AX25_FRAME_U_FINAL);
+    return reply_dm(iface, frame, PATTY_AX25_FRAME_FINAL);
 
 error_sock_delete:
     return -1;
diff --git a/src/sock.c b/src/sock.c
index 9bc562d..3f16f05 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -204,7 +204,7 @@ error_toobig:
 ssize_t patty_ax25_sock_send_sabm(patty_ax25_sock *sock, int poll) {
     return patty_ax25_sock_send(sock,
                                 PATTY_AX25_FRAME_U_SABM
-                              | PATTY_AX25_FRAME_U_POLL,
+                              | PATTY_AX25_FRAME_POLL,
                                 NULL,
                                 0);
 }
@@ -212,7 +212,7 @@ ssize_t patty_ax25_sock_send_sabm(patty_ax25_sock *sock, int poll) {
 ssize_t patty_ax25_sock_send_sabme(patty_ax25_sock *sock, int poll) {
     return patty_ax25_sock_send(sock,
                                 PATTY_AX25_FRAME_U_SABME
-                              | PATTY_AX25_FRAME_U_POLL,
+                              | PATTY_AX25_FRAME_POLL,
                                 NULL,
                                 0);
 }
@@ -220,7 +220,7 @@ ssize_t patty_ax25_sock_send_sabme(patty_ax25_sock *sock, int poll) {
 ssize_t patty_ax25_sock_send_disc(patty_ax25_sock *sock, int poll) {
     return patty_ax25_sock_send(sock,
                                 PATTY_AX25_FRAME_U_DISC
-                              | PATTY_AX25_FRAME_U_POLL,
+                              | PATTY_AX25_FRAME_POLL,
                                 NULL,
                                 0);
 }